Human respiratory syncytial virus (RSV) is a negative-sense, single-stranded RNA virus of the family Paramyxoviridae, which also includes common respiratory viruses such as those causing measles and mumps. There are two primary RSV subtypes: subtype A and subtype B. RSV replicates in the upper respiratory track and then spreads to the lower airways leading to bronchiolitis or pneumonia. The virus causes inflammation, edema of the airways, increased mucus production, and breakdown of respiratory epithelium.
An estimated 64 million cases of respiratory illness and 160,000 deaths worldwide are attributable to RSV-induced disease. Severe RSV infection occurs most often in children and infants, especially in premature infants. Underlying health problems such as chronic lung disease or congenital heart disease can significantly increase the risk of serious illness. RSV infections also can cause serious illness in the elderly, individuals with chronic pulmonary disease and in immunocompromised adults, such as bone marrow transplant recipients.
Several approaches to the prevention and treatment of RSV infection have been investigated. Intravenous immunoglobulin (RSV-IGIV; RESPIGAM®) isolated from donors, and the monoclonal antibody palivizumab (SYNAGIS®) have been approved for RSV prophylaxis in high-risk premature infants. A vaccine or commercially available treatment for RSV, however, is not yet available. Only ribavirin, a RNA inhibitor, is approved for treatment of RSV infection. In order to be effective for treatment of RSV infection, high doses, repeated administrations and/or large volumes of antibody products, such as palivizumab, are required due to low effectivity.
RSV has two major surface glycoproteins, F and G. The F protein mediates fusion, allowing entry of the virus into the cell cytoplasm and facilitating the formation of syncytia in vitro. The F protein sequence is well (˜90%) conserved among RSV strains (Johnson and Collins, J Gen Virol. (1988) 69: 2623-2628). The sole marketed monoclonal antibody palivizumab is directed against the F protein of RSV.
The G protein of RSV is a surface protein that is heavily glycosylated and functions as the attachment protein. In contrast to the F protein, the G protein is quite variable across strains except for a central conserved domain (CCD), comprising amino acid residues 153-184 of the G protein of the RSV A2 strain, or corresponding amino acid residues in other strains. Both the central conserved domain and adjacent regions (residues 145-193) are bounded by rigid and heavy O-glycosylated mucin-like regions. The N-terminal half of the central conserved domain contains a small region that is conserved among more than 700 strains. The C-terminal half contains 4 conserved cysteines that are connected in a 1-4, 2-3 topology and folds into a cystine noose.
Although passive immunization using antibodies directed to the G protein has generally been considered impractical due to the lack of sequence conservation across strains, neutralizing monoclonal antibodies binding to the RSV G protein are known. Anderson, L. J. et al (J. Virol. (1988) 62:4232-4238) describe the neutralization ability of mixtures of F and G murine monoclonal antibodies, one of which binds to the RSV G protein (i.e., 131-2G). The antigenic site of this antibody was later defined by Sullender (Virol. (1995) 209:70-79). This antibody was found to bind both RSV groups A and B, representing the major strains of RSV. In addition, WO 2009/055711 discloses antibodies, such as 3D3 and 3G12, which are immunoreactive with a conserved motif within the G protein of RSV A2 and have neutralizing activity against RSV A and B subtypes. These antibodies have been shown to recognize linear epitopes in the central conserved domain, but have not been tested in the preferred animal model (i.e., cotton rats) for evaluating RSV antibodies and vaccines.
In view of the severity of the respiratory illness caused by RSV, in particular in young children and in the elderly, there is an ongoing need for effective means to prevent and treat RSV infection.